The preparation of a dissolution processing solution containing a low concentration of radioiodine has been discussed by the International Atomic Energy Agency in a thesis entitled "Improved Procedures for efficient iodine removal from fuel solutions in reprocessing plants," IAEA-SM-245/16 (1980), pp. 139-156, especially in items 3.2 and 3.3 thereof.
In general, in a case where spent nuclear fuel whose principal component is uranium dioxide is dissolved in nitric acid at 2-7 mol/liter, a substantial amount of nitrogen oxides are produced according to the reaction formula of: EQU 2UO.sub.2 +6HNO.sub.3 .fwdarw.2UO.sub.2 (NO.sub.3).sub.2 +NO+NO.sub.2 +3H.sub.2 O
Iodides in a dissolution processing solution are readily oxidized into elementary iodine by nitrous acid which exists in an equilibrium relation with nitrogen oxides in the dissolution processing solution, as follows: EQU 2H.sup.+ +2I.sup.- +2HNO.sub.2 .fwdarw.2NO+I.sub.2 +2H.sub.2 O
On the other hand, the elementary iodine is slowly oxidized in the nitric acid to change into a nonvolatile substance such as iodate: EQU 5HNO.sub.3 +I.sub.2 +H.sub.2 O.fwdarw.2H.sup.+ +2IO.sub.3.sup.- +5HNO.sub.2
While the nuclear fuel material is being dissolved in the nitric acid, the nitrous acid exists in the dissolution processing solution, and hence, radioiodine is held in a volatile form. When the uranium dioxide no longer exists in the dissolution processing solution, the nitrous acid in the heated nitric acid is rapidly decomposed to vanish, so that the elementary iodine is likely to be oxidized into the nonvolatile iodate.
The oxidation rate of the elementary iodine in the boiling nitric acid of 2-7 mol/liter is in a direct proportion to the concentration of iodine, and practically, it is about 0.1-1% per minute.
Elementary iodine dissolved in an aqueous solution shifts more easily into its vapor phase as the concentration and temperature thereof are increased. The emission of dissolved elementary iodine may be promoted by blowing a gas into the liquid phase, and the amount of the emission increases depending upon the amount of the gas. When the temperature of the solution is high, the required amount of the gas can be decreased, and ultimately, when water is boiled, the elementary iodine can be expelled by water vapor. Insofar as iodine in the aqueous solution exists in the elementary form, the iodine concentration of the dissolution processing solution can be lowered from 10.sup.-3 -10.sup.-4 mol I.sub.2 /liter (26-260 ppm) to 10.sup.-6 mol I.sub.2 /liter (0.26 ppm) by the vaporization of 10% of the whole volume of solution.
In a conventional dissolution process, elementary iodine is expelled from a dissolution apparatus by a mixture which consists of NO.sub.x and water vapor. Usually, the rate of boiling is raised after the end of the dissolution so as to expel the iodine before the iodine becomes a non-volatile form.
The above prior art is applied to a spent-fuel dissolution apparatus of the so-called batch-process type, and an example of the procedure is carried out as follows:
(1) The dissolution apparatus is loaded with nitric acid. PA0 (2) A basket packed up with nuclear-fuel chops is put in. PA0 (3) By raising the temperature of the nitric acid, the dissolution of a spent fuel material is started and is then accelerated. PA0 (4) While nitric acid is being added into a dissolver, it is boiled to exhaust vapor. PA0 (5) After the end of the dissolution, residual iodine is expelled by the boiling and the vapor exhaust. PA0 (6) After being cooled, the dissolution processing solution is drawn out of the apparatus. PA0 (1) A container for a dissolution processing solution is filled with dissolution processing at all times. PA0 (2) Nuclear-fuel chops are put in the container at fixed time intervals. PA0 (3) Nitric acid is continuously supplied, and the dissolution processing solution is continuously drawn out. PA0 (4) The temperature of the solution is held at a fixed point at all times.
Meanwhile, in the continuous dissolution system to which the present invention is directed, an example of the procedure is as follows:
In the continuous dissolution system, the dissolution processing solution is continuously fed with radioiodine which is continuously produced by dissolution of the nuclear-fuel chops. Further, the dissolution reaction of a nuclear fuel material proceeds in at least a portion of the dissolution processing solution-container at all times. NO.sub.x is accordingly generated, and nitrous acid exists in equilibrium with the NO.sub.x. Therefore, the oxidation of the iodine into iodate may be prevented to a considerable degree. In case of the continuous dissolution system, the radioiodine is normally supplied continuously into the dissolution processing solution by the dissolution of the nuclear-fuel chops, while at the same time, the dissolution processing solution mingled with the nuclear-fuel chops at random is taken out continuously. Thus, whereas in the batch-process dissolution apparatus, 99% of the radioiodine emitted from the spent nuclear fuel is removed from the dissolution processing solution after the final process for expelling the iodine, the removal percentage becomes approximately 95% in the continuous dissolution apparatus.